The present invention relates to the production of high density powdered metal parts and more particularly to a process of forming the powdered metal part to a density of 99+% of theoretical density.
The molding of metal powders has been extensively employed in the production of complicated shapes of soft metals, particularly iron and low carbon steels. The method usually employs a fine metal powder which is pressed or compacted under high pressure to cold weld the metal particles together and then sintered at a high temperature sufficient to form a coherent solid article. Powder metallurgy is currently used for the production of parts that do not require the strength and ductility of wrought steel. In many cases, the tolerances of a powder compact that is pressed and sintered can be held close enough so that no final machining is required; while in other cases, close tolerances can be maintained by coining the parts after sintering. The use of powder metallurgy processes for forming metal articles of various shapes and types is a preferred method of manufacture wherever possible in view of the rapidity of the manufacturing process, its relative simplicity, and the relatively low cost involved. If the mechanical properties of the parts could be improved, the area of usefulness of powder metallurgy in the production of steel parts would be greatly expanded.
The utility of powdered metal articles produced by pressing and sintering frequently depends upon the fact that their physical properties, especially their strength, conform or approach as far as possible to the properties of parts produced from a fused mass. The physical properties of sintered metal articles are influenced to a considerable extent by the production process. The primary cause of the low strength of powder metallurgy steel is the high level of porosity. Typically, a part made from steel powder with a single pressing operation and sintering will be 85% dense (15% porosity). Porosity can be reduced by repressing and resintering but porosities of less than 7% are difficult to achieve and are economically impractical. Only at low pressures and low densities does an increase of the pressure also bring a proportional increase of the density. At higher pressures and higher densities, on the other hand, an increase of the pressure leads only to a relatively slight increase of the density. This is attributable to the fact that in the pressing of metal powders, a cold work-hardening occurs which increases the deformation resistance of the powder particles, and thus slows the compressing operation, and finally brings the latter to a halt. For this reason, it is difficult to produce sintered parts of high density with pressures at which tool wear and tool breakage are kept within economically acceptable limits.
Further densification has also been achieved by a hot pressing operation. The powder is loaded into a hot die and pressed, however, the method is slow because of the long time required to heat the powder and, therefore, is economically feasible only for expensive materials. As powdered metal components usually have a complex geometry, such as gear teeth, splines, hubs, webs, etc., that are not capable of forming by the simple fabricating processes such as rolling, drawing or swaging, and as these components are made in extremely large quantities and must be interchangeable, it is important that any process used for such fabrication be capable of making parts repeatedly within very small dimensional tolerances and with uniformly high densities. The present invention overcomes the deficiencies of prior known processes in providing a finished or nearly finished powdered metal part having a density of 99+% of theoretical density.
Among the objects of the present invention is the provision of a process for making powdered metal parts into finished or nearly finished, high-strength, structural steel parts of complex configurations. This method includes the basic steps of cold forming a suitable blend of powdered metals into a coherent body or preform having a prescribed density, thermally treating the preform to achieve prescribed chemical and metallurgical properties, transferring the preform at an elevated temperature into a temperature-maintained die, and forming the preform under relatively low pressure into a finished or nearly-finished high density part.
Another object of the present invention is the provision of a powder metallurgy process wherein the starting material is a prealloyed steel powder that is blended with graphite and a suitable lubricant and then preformed into a compact approaching the shape of the finished part. The density of the preform is limited to approximately 80% of theoretical to insure that the pores of the preform are mostly interconnected. The amount of graphite added to the metal powder must be sufficient to reduce the oxides therein and yet bring the final carbon content of the part within .+-. 0.05% carbon of the desired final carbon content.
A further object of the present invention is the provision of a process for forming powdered metal wherein the preform is thermally treated at an elevated temperature to reduce the oxygen content of the preform to 300 parts per million or less. The preform at the elevated temperature is then preferably directly transferred to a hot pressing die with the transfer time minimized to avoid reoxidation or decarburizing the surface of the compact. The rapid transfer of the preform also accomplishes a minimal heat loss of the preform so that the final densification of the article is accomplished at an elevated temperature near the thermal treatment temperature.
The present invention relates to a process of hot densification of a powdered metal preform in a hot pressing die at a relatively low pressure. To achieve the hot densification of the powdered metal preform, the die is preheated to a temperature within the range of approximately 1000.degree. to 1400.degree. F, and the temperature of the transferred preform is approximately 1950.degree. F. The forming pressure for the hot densification is in the range of 19 to 39 tons per square inch, and the die composition for the hot densification is a high nickel-based alloy to reduce the wear and breakage thereof during repeated pressing operations. The final temperature of the ejected, finished or near finished part is approximately 1500.degree. F, and after pressing, the powdered metal article is ejected from the die and transferred to a container in which it can immediately be cooled by a liquid quench, such as oil or water, or in an inert atmosphere, such as nitrogen, to prevent the part from becoming oxidized before it is cooled to a sufficiently low temperature. The density of the final finished part exceeds 99% of theoretical density. In some cases, it may be necessary to perform additional operations in order to bring the part to its final geometry and physical characteristics. Secondary operations include grinding, if extremely close tolerances are required, or transverse holes or undercuts may be machined into the part which cannot be done in the forming operation. Additionally, the part may be carburized or heat treated if such treatment is required to meet the final physical properties in the body and surface of the part.
Further objects are to provide a construction of maximum simplicity, efficiency, economy, and ease of operation, and such further objects, advantages and capabilities as will later more fully appear and are inherently possessed thereby.